Everything you ever wanted to know about particle smashers (but were afraid to ask)

Can't tell RHIC from ALICE? Wonder how we can detect dark matter when it's, …

On March 30th, humanity is scheduled to start running the biggest, baddest particle collider ever constructed, the one that makes its closest competitor, Fermi's Tevatron, so out-of-date that the current plan is simply to shut the Tevatron down. But Brookhaven's RHIC, which can't even reach the Tevatron's energies, will be kept running indefinitely.

If that doesn't make sense, it's just one of a large collection of things associated with particle physics that may not be very intuitive. Like the fact that we've built a series of detectors in the Large Hadron Collider that are designed to pick up signs of particles that we've never seen before, and tiny bits of dark matter that don't actually interact with anything. Or the fact that we need a bunch of detectors for this in the first place. Fortunately, for the last month or so, I've had a variety of physicists explaining matters to me in language that even a biologist could understand. In a series of articles, I'll try to explain the process of physics so that, as the LHC gears up, you can appreciate why scientists think it's a pretty big deal.

The recent generation of particle accelerators rely on essentially identical principles: take an atom, strip off all its electrons, and use the charge that remains as a lever to boost it to speeds that are a rounding error away from the speed of light. Two beams of identical particles are sent around a ring-shaped accelerator in opposite directions and, at specific points, the beams are made to cross, creating the possibility of collisions. For the LHC, this primarily involves the simplest atomic nucleus, the single proton of hydrogen. RHIC uses gold, with 79 protons and 90 neutrons, although the LHC is capable of using lead, which is similar in mass to gold. (We'll get into the whys of the two weights shortly.)

The atoms end up going so close to the speed of light that distance contracts along the path of their travel—if you could watch the gold atom shoot past from the side, it would look like a disk. (In fact, Brookhaven scientists had to change their images from spheres to disks after complaints from anal-retentive audience members [my term, not theirs].) It's moving so fast that the huge repulsion that might otherwise result from the large collections of positive charges doesn't even have a chance to slow things down. "By the time these guys realize they should experience charge repulsion," said Brookhaven's Peter Steinberg, "it's all over."

Different generations of hardware exist side-by-side in the control room at the Relativistic Heavy Ion Collider

Two protons bring a total of six quarks into the collision, yet a large collection of particles results from their collision. Two gold ions bring in over 150 protons and even more neutrons, and over 10,000 particles come streaming out—physicist Todd Satogata said that, to scale, it looks "like a Kush ball the size of a three-story house." Where's all this stuff coming from?

When particles collide, they bring a tremendous amount of energy to a screaming halt. It all has to go somewhere, and some of it ends up radiating off as massless photons. But, because mass and energy are equivalent, a certain amount of the energy gets converted directly to mass and kinetic energy, creating the particles that spray from the collisions.

Why the LHC is a big deal

Here's where we get into the difference between RHIC's use of gold and the LHC's use of protons. Since energy and mass are proportional, the heavier the particle you hope to create, the more energy you have to pump into the collisions. So, for example, if a particle's mass is the equivalent of a trillion electron volts, you have to have collisions with at least that much energy to have any chance of producing it. As the energy involved in the collisions increases beyond this point, you have an increasing chance of producing the particle in question.

For well over a decade, we've been stuck with collisions that are just under 2TeV, provided by Fermi Lab's Tevatron. That was easily enough to produce the top quark, and may be within the range of a major hypothetical particle, the Higgs boson. But the probabilities of spotting the Higgs in the Tevatron are pretty low, which means that long runs and lots of data would be needed to know for sure.

At its maximum energy, the LHC will produce collisions that are nearly an order of magnitude more powerful than the Tevatron's, and it will produce a lot more of them—it has a higher luminosity, as physicists put it. The Higgs will be easily within reach, and any number of more exotic hypothetical particles should definitely be possible.

What's the point of a low-powered collider like RHIC?

At its highest energies, Brookhaven's Relativistic Heavy Ion Collider couldn't even produce a top quark. Why's it considered so valuable that it'll keep going after the Tevatron's scheduled to shut down? As Brookhaven's Steinberg put it, the LHC is a particle machine, but RHIC is a quantum chromodynamics machine—it studies the interactions among fundamental particles when they're present in bulk. The LHC can do that as well, with its ability to accelerate lead and an entire detector, ALICE, devoted to studying these collisions. But that wasn't what it was built for, so new physics like the Higgs and dark matter particles will always be a higher priority.

RHIC's top priority has always been the nuclear program. Although the collisions of heavy ions also produce a fair share of unusual particles like strange quarks, their key feature is the fact that they take lots regular quarks, which are normally bundled in the protons and neutrons of the nucleus, and pack them into an incredibly dense space at extremely high energy. Under these conditions, the forces that keep quarks and gluons neatly bundled into these particles break down, and they diffuse into a dense particle soup. That soup lasts a total of 30 yoctoseconds—long enough for light to cross the width of about 10 protons—before escaping in a miniature fireball.

This quark-gluon plasma hasn't been seen with any regularity since shortly after the big bang. In fact, Brookhaven scientists were fairly cautious about claiming that they had even created it. Even though physicists had been talking as if it had existed for years, the paper that provided the best evidence to date was only published this year.

There are two interesting aspects to these collisions. The first is the behavior of the quark-gluon plasma, which appears to act as a perfect quantum liquid, with properties that Brookhaven staff suggested made the odd behavior of Bose-Einstein condensates look positively tame. The other is that, as the quarks rocket out of the collision site, they will re-condense into familiar (and strange) nucleons. If enough of these end up travelling in close proximity, they can also condense into a regular nucleus (or antinucleus). These processes can help us understand how the world of regular matter gets put together.

RHIC also does some collisions using protons to study phenomena that may not require the huge energies available at the LHC. One of these involves a search for the proton's spin, which is the sum of the spins of its component quarks. That may sound nice and intuitive, but there's apparently nothing intuitive about the physical process that performs that addition. Intriguingly, the search for source of the spin has come up short so far. "We've figured out a lot of places it isn't," said Steinberg, who called it "the most fascinating null result I'm aware of."

We're arranging to talk to someone about the proton experiments. In the meantime, check in tomorrow as we discuss how the huge detectors at the LHC and RHIC manage to identify the tiniest of particles, and use that information to reconstruct what happens in the instants after collisions take place.

Ars Science Video >

A celebration of Cassini

A celebration of Cassini

A celebration of Cassini

Nearly 20 years ago, the Cassini-Huygens mission was launched and the spacecraft has spent the last 13 years orbiting Saturn. Cassini burned up in Saturn's atmosphere, and left an amazing legacy.

All I want to know is if this is another one of those silly experiments that could blow up the only planet we have to live on... yea I know "stupid paranoid technopobe it's 50 million to 1 that something bad would happen" but why can't they wait a few decades and plonk the damn things on an asteroid somewhere far away from me.

All I want to know is if this is another one of those silly experiments that could blow up the only planet we have to live on... yea I know "stupid paranoid technopobe it's 50 million to 1 that something bad would happen" but why can't they wait a few decades and plonk the damn things on an asteroid somewhere far away from me.

I think it's more proper to say "why high energy physicists think..." I remain unconvinced that anyone outside that field really has any investment in it at all. Many are openly hostile. And if one is jaded enough, and I am, then the answer is "because we don't have anything else to do".

In the early days of HEP the field was wide open and even small experiments had a good chance of turning up new particles. Those particles had properties that were novel and continued to turn any understanding we thought we had on its head, over and over. The experiments told us the physics.

Then, with the development of QCD and its extension to the Standard Model, all of that went away. Now we had a theory that was more powerful than the experiments. From that point on, in the 70s, the experiments have been slaves to the theory. Since the cost of building the smashers was growing into the nine digit range, you had to be sure that it would detect something. So you picked one of the particles in the SM that hadn't been seen yet, and spent a whole lot of money to build a machine who's only purpose in life was to find it...

ICE @ CERN - W and ZTevatron - Top quarkLHC - Higgs

But here's the problem: since the SM was well established, there's been NO MORE PHYSICS. None. The last 25 years of HEP hasn't turned up a single new bit of physics. One might argue that they can't, these machines, and the experiments we run on them, are so finely tuned that anything novel is lost as noise that's deliberately filtered out. (see any number of papers on the topic, and most definitely Constructing Quarks)

The bigger problem is that we already know the SM is wrong - or at least incomplete. While the accelerators nerds were coming up with ways to try to get another order of magnitude in energy (for another order of magnitude in cash) the telescope people went four orders of magnitude for one in price in 1/4 the time. In the last decade we've built a series of telescopes that have made more measurements than every telescope in existence over the last century (probably by a couple orders of mag).

Those experiments are repeatedly demonstrating that our understanding of the universe is wrong. Or not wrong so much as incomplete. Nothing that LHC could find explains dark energy, the large scale structure of the universe, gravity, dark energy or many other issues. Additionally, many of the techniques used for then highly automated sky-searches are also very useful for studying HEP, by building arrays of inexpensive telescopes that look for cosmic ray interactions. A number of experiments along these lines are being set up now.

If you want to put your bet on which field is going to turn up real physics, it's telescopes all the way.

Now it's been argued that LHC isn't really being built just to detect Higgs - as one editor argued here recently. That's true. If we get very lucky, then the lowest energy supersymmetric partners might be in an energy range we can detect in LHC. If that's true, then we are indeed lucky, because if that turns out to be the Neutralino then we can claim to have solved dark matter too. Here's the problem: the most likely mass for the Neutralino is between 10 GeV and 10 TeV. Anywhere on the lower side and we would have already seen it. On the upper end, LHC can't see it. In either case, it's very difficult to look over 4 orders of magnitude in any sort of search. Even if LHC does get one, which is good physics, then we're fully aware that the machine we'd need to explore it cannot be built.

For the most part, LHC is an expensive machine to put the cake topper on SM. I simply don't see the value.

They need to ban this technology and consider it a weapons of mass destruction. These idiots are going to end up generating a blackhole and destroy the earth because they want to play god by trying to smash molecules and create mini universes etc..

I never understood why we worry more about global warming which is a BS theory and not focus on this real and present danger of scientists trying to play god.

They need to ban this technology and consider it a weapons of mass destruction. These idiots are going to end up generating a blackhole and destroy the earth because they want to play god by trying to smash molecules and create mini universes etc..

I never understood why we worry more about global warming which is a BS theory and not focus on this real and present danger of scientists trying to play god.

They need to ban this technology and consider it a weapons of mass destruction. These idiots are going to end up generating a blackhole and destroy the earth because they want to play god by trying to smash molecules and create mini universes etc..

I never understood why we worry more about global warming which is a BS theory and not focus on this real and present danger of scientists trying to play god.

They need to ban this technology and consider it a weapons of mass destruction. These idiots are going to end up generating a blackhole and destroy the earth because they want to play god by trying to smash molecules and create mini universes etc..

before the first actual nuclear detonation, it was a fear that it could light the atmosphere on fire.

but as it was a war going on, and at the time it was not known that the enemy had fouled up their research so bad it could never work, things where pushed ahead anyways.

An upgrade to the LHC is already planned after, I believe, 10 years of operation. I'm certainly not qualified to debate whether that money is well spent, but I don't think it would go to other research if it weren't spent on this. CERN is an international effort whose funding is as best as I can tell the result of treaties, not of domestic budget negotiations. $8bn split between 30 countries isn't exactly significant spending.

I would be cautious about debating the "usefulness" of spending. Let's be honest: neither the LHC nor most of astronomy has any medium-term impact on the world. Never mind pure mathematics, which would be out of funding immediately. Maybe in 50-100 years some of this will actually have an application - but that's a lot of spending on a maybe. That being said, I still think it's extremely worthwhile spending - certainly better than just about any alternative. I just don't think we should consider approaches other than one's own as "wasteful" or ineffective.

Quote:

The bigger problem is that we already know the SM is wrong - or at least incomplete.

Isn't it likely, then, that they would find more than just the Higgs? I thought I read a brief blog post from someone working at the LHC that they found much more events than they expected. I can't seem to find it anymore, though. In any case, there is a conference upcoming in summer and I'd expect the LHC people to present some of their findings there.

I don't really get that section. It seems starts off saying that the LHC is "better" than the RHIC because it uses protons instead of gold, and that this is because more energy = better. I doesn't seem to explain why smashing protons means more energy than smashing gold. Intuitively, that seems backwards. Help?

Maury, that's an interesting argument. One that would make any economist or astronomer proud.

But here's the problem. Theories are great, but they still need to backed up with experimentation. I'm assuming you're a scientist, thus you should know that. Additionally, out of that experimentation often new questions arise and new theories are born. It sounds like the quark-gluon plasma and proton spin experiments at RHIC fall under that umbrella.

So while it does cost vastly larger sums of money to build and operate the particle smashers, the knowledge and further questions that they bring up is invaluable still.

And then of course there's the obvious argument that we have to look at the universe at both macro and micro levels. Sacrificing one for the other could result in us missing a huge body of knowledge.

So be happy. The LHC got built and so did the telescopes. Getting rid of either of those would be a shame.

So does that mean that the gold in the RHIC doesn't reach the same speeds as the protons in the LHC will?

well it starts out heavier, so it may well require more energy then the proton of LHC. Note however how its also mentioned that the LHC can also make use of lead.

i think its a tradeoff. heavier atoms results in higher energy requirements, but at the same time may provide more data to analyze once the collision is a fact (more particles that have a chance to interact). Still, it also produces a whole lot more noise that need to be sorted.

The faster and faster a mass of particle gets the heavier and heavier it becomes, at light speed the weight of a proton is infinite mass.

That's one way of looking at it, although not always strictly correct, it is a good simplification. It depends on how you define "mass" but basically they gain tremendous amounts of energy and momentum so you could interpret it as increasing mass.

Maury, that's an interesting argument. One that would make any economist or astronomer proud.

But here's the problem. Theories are great, but they still need to backed up with experimentation. I'm assuming you're a scientist, thus you should know that. Additionally, out of that experimentation often new questions arise and new theories are born. It sounds like the quark-gluon plasma and proton spin experiments at RHIC fall under that umbrella.

So while it does cost vastly larger sums of money to build and operate the particle smashers, the knowledge and further questions that they bring up is invaluable still.

And then of course there's the obvious argument that we have to look at the universe at both macro and micro levels. Sacrificing one for the other could result in us missing a huge body of knowledge.

So be happy. The LHC got built and so did the telescopes. Getting rid of either of those would be a shame.

i suspect the basic problem is that with the cold war "gone", there is less willingness for the military or others to perform blue sky research. They want research that can be monetized quickly. And right now physics do not produce that. The last time it did may well have been the laser. But then the laser started out as a test of a theory, and at first had no known practical applications. But now we have fiber optic communications, optical storage media, and pinpoint surgery and material work.

it can also be a case of humanity having become a impatient lot, expecting "miracles" to show up every 5 years or so. That, alongside a potential loss of faith in science to come up with safe solutions (thanks to DDT, plastics and nuclear waste), may make people question the funding of projects of unclear worth and endpoint.

"The equation E = mc2 indicates that energy always exhibits mass in whatever form the energy takes.[3] It does not imply that mass may be “converted” to energy, for modern theory holds that neither mass nor energy may be destroyed, but only moved from one location to another."

RHIC is usually described in terms of energy per nucleon. Each gold ion has about 200 nucleons (protons and neutrons). Thus, the total per-nucleus energy of the RHIC is far in excess of the LHC. However, at the energy scale of modern accelerators you can think of the nucleons (and indeed the quarks within each nucleon) as free particles. The energy per nucleon is what governs interactions. So the RHIC generates a large soup of slower collisions compared to the LHC.

"The equation E = mc2 indicates that energy always exhibits mass in whatever form the energy takes.[3] It does not imply that mass may be “converted” to energy, for modern theory holds that neither mass nor energy may be destroyed, but only moved from one location to another."

That Wiki quote is either complete BS or poor wording, depending on how you look at it. Both a positron and an electron have mass, yet they very commonly annihilate each other to produce a couple of 0.511MeV photons, which have no mass.

All I want to know is if this is another one of those silly experiments that could blow up the only planet we have to live on... yea I know "stupid paranoid technopobe it's 50 million to 1 that something bad would happen" but why can't they wait a few decades and plonk the damn things on an asteroid somewhere far away from me.

"But, because mass and energy are equivalent, a certain amount of the energy gets converted directly to mass and kinetic energy, creating the particles that spray from the collisions."

Is it true that energy can be converted to mass?

Yes, energy and mass are inter-convertible. In most processes that release energy the difference in mass is too small to be readily detectable (most chemical reactions fall into this category). However, in nuclear reactions (fission, fusion, and especially matter-antimatter reactions) mass is converted directly to energy. The mass of two moles of deuterium is 4.028 grams, while the mass of the resulting mole of helium is 4.003 grams; the mass difference of 0.026 grams is released as energy (in large amounts), which is why nuclear fusion is of interest as an energy source (and is a part of the large energy release in thermonuclear weapons, although other nuclear processes that also convert mass to energy are more relevant than D+D fusion).

All I want to know is if this is another one of those silly experiments that could blow up the only planet we have to live on... yea I know "stupid paranoid technopobe it's 50 million to 1 that something bad would happen" but why can't they wait a few decades and plonk the damn things on an asteroid somewhere far away from me.

"Another one"? How many silly experiments have we done that's blown up the world?

First, can the LHC blow up the world? If smashing high-energy particles together could blow up the world, we'd already be gone, because the earth is regularly bombarded by high-energy cosmic rays, with energies the LHC cannot achieve. Well, what if it makes a mini-black hole? Would it swallow the earth? Well, despite what happened to Vulcan in the new Star Trek, atomic-sized black holes don't grow, they lose mass-energy to Hawking radiation so they act like any other unstable particle. What if there's unknown physics that makes something horrible happen? Well, if so, that unknown physics is already being explored by cosmic rays, so we'd be doomed anyway.

Second, do scientists think that something from the future is interfering with our ability to detect the Higgs boson? No. One scientist published an article suggesting the idea, and everyone else thinks it's crap, but the media ran with it.

Idea for another article: Describe what's different about the planned International Linear Collider http://www.linearcollider.org/ and why it wouldn't compete with or replace either the LHC or RHIC.

If it's built the ILC will collide electrons (e-) and positrons (e+) both of which are elementary particles. It's much easier to work out what happened after the collision of two elementary particles since they're both effectively single points, whereas protons are made up of several quarks, and thus collisions between protons (and even more so between Gold nuclei) involve the quark soup that this article talks about. It's like the difference between firing two shotgun cartridges at each other (LHC, RHIC) versus two rifle bullets (ILC).

I'm happy to see intermediate energy particle physics presented beside the more glamorous world of high energy physics in the media for once. I'm a bit disappointed that this article is so short and so Brookhaven centric (as it is billed at "Everything You Wanted to Know"), but it's welcome nonetheless. I'd be careful about the dismissive talk of the Tevatron, though. I just saw a colloquium that claimed that Tevatron data that is still being combed through has a good chance of finding the Higgs before the LHC does!

I hope to see that dangling line about spin is followed up on. It's a bit misleading. Of course, for nucleon spin structure you should also talk to someone at JLab or DESY... Or send an Ars writer to Julich, Germany this fall for the Spin 2010 conference! http://www.fz-juelich.de/ikp/spin2010/en/index.shtml

All I want to know is if this is another one of those silly experiments that could blow up the only planet we have to live on... yea I know "stupid paranoid technopobe it's 50 million to 1 that something bad would happen" but why can't they wait a few decades and plonk the damn things on an asteroid somewhere far away from me.